Pressurized-water nuclear reactors generally comprise a vessel enclosing the core of the reactor which is immersed in the pressurized cooling water of the core of the reactor.
The vessel of the reactor of overall cylindrical shape comprises a head of hemispherical shape which may be attached onto its upper part. The head is pierced with openings in the region of each of which there is fastened, by welding, a tubular penetration piece constituting an adaptor providing the passage for and controlling the movement of an extension of a control cluster for the reactivity of the core or a penetration passage for a means for measurement inside the core, such as a thermocouple column.
To the end parts of each of the adaptors, there are fastened mechanisms for controlling movement of the control clusters for the reactivity of the core.
Inside each of the tubular penetrations of the vessel head there is fastened, in a position which is coaxial with respect to the tubular penetration piece and with a certain radial clearance, a thermal sleeve which comprises a diametrically flared part coming to rest on a diametrically flared bearing surface located at the upper part of the bore of the tubular penetration piece and which is mounted to rotate freely inside the penetration piece.
The extensions of the rods for controlling the reactivity of the nuclear reactor pass through the vessel head inside thermal sleeves which are themselves arranged coaxially inside adaptors for the control rods or more generally inside tubular penetration pieces of the head.
In order to increase the reliability and operational safety of nuclear reactors and to extend the durability of these reactors, plant operators are led to carry out more and more numerous inspections of the various elements making up the nuclear reactor.
In particular, it may be necessary to monitor the state of the penetration pieces of the head of the vessel in order to be sure of the integrity of these pieces after a certain operational time of the reactor, in particular in the zone where these tubular pieces are welded onto the vessel head.
This monitoring, which must make it possible to detect and to repair defects on the internal surface of the tubular penetration piece, must be carried out from inside the bore of the tubular piece and may consequently necessitate dismantling the thermal sleeve to gain access to the inside surface of the penetration bore.
Monitoring and repairs are carried out during a shut-down of the nuclear reactor, the head of the vessel having been dismantled and placed at the level of an intervention stand.
The dismantling of the thermal sleeve requires complex operations, insofar as the flaring of the thermal sleeve resting on a bearing surface arranged at the upper part of the adaptor prevents the dismantling of the sleeve by pulling downwards, on its lower part which is accessible below the head.
Furthermore, it is also impossible to dismantle the sleeve by pulling upwards, insofar as the cluster control mechanisms which are fixed by screwing and by welding onto the upper parts of the adaptors prevent the passage of the thermal sleeve.
It is therefore desirable to limit the complex dismantling operations of the thermal sleeves to those adaptors which effectively exhibit defects requiring repair.
The operations for monitoring penetration pieces from inside their bore may be carried out effectively by using monitoring probes such as ultrasound probes or Eddy current probes insofar as these probes may be placed opposite or in contact with the inside surface of the bore of the penetration piece.
In French Patent Application No. 92 02405 filed Feb. 28, 1992 by FRAMATOME and Electricite France, it was proposed to make at least one longitudinal slit through the wall of the sleeve by machining the sleeve from the inside, to insert a monitoring probe into the sleeve level with the slit, and to monitor by moving the thermal sleeve in rotation about its axis inside the tubular piece and by moving the probe in translation along the length of the slit.
It was also proposed to effect the repair of the tubular piece such as an adaptor in the event of defects being detected, through the slit made in the sleeve.
Such a method presents numerous advantages with respect to the method requiring dismantling of the thermal sleeve but it nevertheless remains necessary to machine the sleeve in order to make the slit.